Project Summary/Abstract Advances in genome sequencing technologies over the past decade have provided a comprehensive survey of the somatic mutations that contribute to cancer. In colorectal cancer (CRC), the 3rd most common cause of cancer-related death in the United States, genome-wide sequencing identified APC, KRAS, and PIK3CA as three of the most commonly mutated genes. Nevertheless, large-scale, genome-wide sequencing efforts do not provide sufficient granularity with respect to the interactions between mutant genes, especially at the level of specific mutant alleles. Precision medicine, where a physician tailors a patient's therapy to the genes that are mutated in his/her cancer, requires this level of understanding because the activation state of oncogenic signaling pathways targeted by precision medicines is dependent upon the panoply of genetic changes in a cancer rather than on mutations in individual genes. A case-in-point of this concept relates to KRAS, in which activating missense mutations occur in 40% of CRCs. Among those 40% of CRCs, the diversity of KRAS mutations is greater than in any other type of cancer. We hypothesize that CRCs might select for KRAS alleles that are absent or rare in other cancers because of a genetic interaction with mutant APC, which is nearly ubiquitous in CRC, but rarer in other cancers. Moreover, aside from APC, the gene most commonly co-mutated with KRAS is PIK3CA, yet PIK3CA mutations co-occur only with specific alleles of KRAS. Building upon our expertise in studying the genetics of cancer using genetically engineered mouse models, and based on our extensive preliminary analysis of animals engineered to express mutant forms of K-Ras in the colon, we will perform an in-depth study of genetic interactions between cancer genes in CRC. This work is separated into two specific aims: (1) To determine the molecular mechanism underlying the genetic interaction between APC and KRAS and (2) To understand why PIK3CA mutations occur preferentially with specific KRAS alleles in CRC. In the end, this study will provide key insights into the genetic interactions that occur in CRC and may reveal allele-specific therapeutic approaches for cancers expressing mutant KRAS.